Contactless energy transmission systems based on induction are known in general. The principle is to use one or a plurality of transmitter or primary coils (also known as coils, conductor loops, turns, antennas, or current conductors) to generate voltage in one or a plurality of relatively closely positioned receiver or secondary coils by means of a temporally changeable current flux based on the principle of induction. The temporally changeable voltage induced in this manner in the receiver coil may then for instance be rectified and used to supply an electronic circuit with power and/or to charge a battery. There are numerous applications for this in consumer electronics, wherein mobile devices such as for instance laptops and smartphones are equipped with at least one integrated receiver coil and are to be arranged on a surface equipped with one or a plurality of transmitter coils, such as for instance a table surface, to be supplied energy inductively via the latter. This is then how for instance the accumulators or batteries of the mobile device are charged. Additional applications that employ the principle of induction are for instance radio frequency identification systems (RFID).
In some applications of contactless energy transmission it is advantageous when the transmitter coils or a plurality of transmitter coils are integrated in a planar unit. For instance, thinly wound, printed, or etched coils on or in a multilayer substrate, such as for instance a printed circuit board (PCB), may be used. Such a planar unit may then be integrated in an extremely varied manner into everyday items, such as for instance walls, floors, drawers, tables, etc.
The physically adjacent coils, one or a plurality of transmitter coils and the receiver coil, form a transformer-like coupling. However, compared to conventional transformers, it does not have any core or at least does not have a closed core. Thus it is generally possible to generate differently shaped magnetic field distributions and therefore consequently also to use various magnetic field components for induction. With respect to the planar unit, the horizontal field components (parallel to the surface of the planar unit) or the vertical field components (parallel to the normal vector of the planar unit), relative to the planar unit, are used for the energy transmission. Such systems are known for instance from US 2008/0116847, US 2003/0210106, and U.S. Pat. No. 7,633,263. In addition, it is possible to use suitable wiring of a plurality of receiver coils to exploit both field components (horizontal and vertical). US 2009/0303746 describes this, for instance.
In the case of vertical field components, in one type of application, two identical conductor loops (transmitter and receiver coil) that are arranged parallel and coaxially are used for transmitting the power at a small distance (very much smaller than the dimensions of the conductor loops) with high coupling. In this context, essentially the coupling factor k, which identifies the ratio of counter-inductivity M to the square root of the product of both self-inductances, is used as the coupling for two coils (characterized by the two self-inductivities L1 and L2). It is expressed as:k=M/√{square root over (L1·L2)}Due to the fact that other physical effects, such as for instance capacitive coupling between tracks, must be taken into account that are not adequately described by the above self-inductivity and counter-inductivity, the term “coupling” in the context of this application indicates the electromagnetic coupling between two coils taking into account all physically relevant effects. The coupling is thus a measure for the quality of the transmission path and ranges from 0 (coils not coupled) to 1 (maximum coil coupling). By definition, the coupling between two coils is a symmetrical variable, which means that the coupling from the first coil to the second coil is identical to the coupling from the second coil to the first coil. In this context, the term counter-coupling is also used synonymously with coupling or mutual coupling.
When two identical conductor loops are used as the primary and secondary coils, the coupling is highly dependent on the relative position of the two coils. Thus for instance the original high coupling is no longer maintained if the distance between the two coils is increased or if one of the two coils is displaced laterally. In many applications, however, it is desirable to provide a relatively position-independent coupling and thus consequently position-independent functionality.
In accordance with the prior art, the object of generating the most homogeneous possible field on a larger planar surface and thus achieving the most homogenous possible coupling is attained for instance in that a different size is selected for the transmitter coil than for the receiver coil. Although this initially reduces the maximum achievable coupling, if cleverly designed with a spiral-shaped transmitter coil, as described for instance in US 2008/0278112, it permits a larger horizontal area with a relatively homogeneous coupling. This also permits a relatively simple system design. As an alternative to this, periodic parallel or series antenna structures, as are described for instance in US 2005/0189910 and U.S. Pat. No. 7,164,255, have proven useful. In these documents, many small transmitter coils are interconnected on one or a plurality of layers of the planar unit to create a virtual large transmitter coil such that a magnetic field that is as homogeneous as possible is created and thus the coupling (via the planar unit) with any receiver coil that is present is also homogeneous.
One drawback of these arrangements is that undesirably large electromagnetic stray fields may be created in the near vicinity of the large or virtually large transmitter coil, even if there is no receiver there. This is disadvantageous both for reasons of electromagnetic compatibility (EMC) and electromagnetic environment compatibility (EMEC), especially because of safety aspects relating to interaction with human tissue and potential health hazards.
The problems associated with undesired electromagnetic stray fields may be circumvented if the size of the transmitter coils is small compared to the size of the receiver coils. It is possible to minimize the electromagnetic stray fields as much a possible if only those transmitter coils that are physically disposed in the immediate vicinity of a receiver coil are activated, and if the coils are surrounded by materials that shield, such as for instance ferrites.
Such approaches and solutions are known from US 2007/0182367, U.S. Pat. No. 7,262,700, U.S. Pat. No. 7,521,890, US 2009/023719 A1, US 2010/0314946, US 2010/0328044, US 2011/0025133, and U.S. Pat. No. 7,893,568. These disclose fields (arrays) of switchable transmitter coils on a planar unit, which fields may be individually switched. In these documents, in addition to the relative positioning and arrangement of the fields of coils, the issue of adequate activation and control of the individual coils also plays a major role. However, this aspect is not the subject-matter of the present invention.
The advantage of this technique is that a plurality of receiver coils may be supplied in a relatively simple manner because only the most closely positioned transmitter coil is turned on and supplied with power. Moreover, the aforesaid solution may be scaled in size in a relatively simple manner in that the planar unit is for instance enlarged by adding additional transmitter coils. The critical disadvantage of these solutions, however, is that a suitable switching device must be provided for each transmitter coil, which increases the complexity of the arrangement and the electronics associated with it and therefore increases costs.